The use of solid-state radiation detectors is based on exposing a semiconductor piece to incoming radiation. The interaction of a received photon with the semiconductor material creates a number of electron/hole pairs, also referred to as free charge carriers. At least one species of free charge carriers are accumulated to electrodes connected to the semiconductor material, and the amount of electric charge thus collected is measured. The total amount of collected charge reveals the original energy of the received photon. If the detector has spatial resolution, detecting the location at which the charge cloud was created enables using the measurement results for imaging. The semiconductor piece is often referred to as the crystal, because detectors of best quality require a single-crystal semiconductor piece with as few lattice faults as possible.
Patterning a detector crystal may have various aims. Dividing the detector crystal into discrete pixels enables making an imaging detector, in which the charge collected by each pixel can be read separately. Even if the detector is not meant for imaging, patterns on the crystal surface may be used as guard structures that control and limit the flow of charges in the crystal.
Known methods for patterning a detector crystal include at least photolithography and ion beam based methods. The former refers to a family of processes, the common features of which include depositing a photoresist on a surface to be patterned, selectively hardening the photoresist through exposure to light, and utilizing the chemical differences between hardened and non-hardened photoresist to etch out desired portions of the surface beneath the photoresist. Ion beam deposition may also use a photoresist, but the eventual mechanism of actual pattern production is not chemical etching but ions abrading away the areas not covered by the photoresist. Focused ion beam techniques are also available and can be used for example to directly inject new material onto the surface to be patterned.
The above-mentioned methods are not sufficient for good results in all patterning applications. As an example we may consider the case of FIG. 1, which shows a cadmium telluride (CdTe) detector crystal 101 with a diffused indium layer 102 on its back surface. The front surface has a thin contact layer 103 made of a metal, like gold, platinum or palladium. The terms “front” and “back” are here used in the conventional sense to tell that the radiation to be detected is meant to enter through that surface that has the metal contact layer 103. The thicknesses of the layers are not to scale. In practice, the thickness of the detector crystal 101 could be in the order of millimeters, the thickness of the diffused indium layer 102 could be in the order of tens of micrometers, and the thickness of the metal contact layer 103 could be in the order of nanometers.
It would be relatively easy to add spatial resolution to the detector of FIG. 1 by patterning the metal contact layer 103 with e.g. photolithographic methods. However, using the front surface for patterning would mean that also the readout electronics should be located on the front surface of the detector, where they may interfere with the incoming radiation. The diffused indium layer 102 is typically too thick (in the order of some tens of micrometers) to be patterned with e.g. photolithography.
Basically it would be possible to replace the thick diffused indium layer 102 with a thin metal contact also on the back surface, which thin metal contact could then be patterned. However, this would easily lead to an unacceptably high leakage current in the detector. The PN junction at the interface between cadmium telluride and indium is vital in curbing the leakage current.